For months now, Blizzard's site has listed job openings for people willing to work on a "Next-Gen MMO." And, for all this time, what this MMO would be was a matter of pure speculation.

Now, we can say one thing for sure. Thanks to a Blizzard employee's post on the official World of Warcraft forums, we can say for sure that the upcoming MMO is no mere expansion. The employee's own words are as follows:

"it's an unannounced Next-Gen MMO. And that doesn't mean an expansion for World of Warcraft either."

So, it's still a matter of speculation as to what the new MMO will be, but for now, we can definitely say that it's not WoW-related.